1. Field of the Invention
The present invention is concerned with an apparatus for attaching a rear view mirror to the interior of a motor vehicle. More particularly, the present invention pertains to an apparatus made of a synthetic resin for attaching a rear view mirror proximate the driver's seat in such a way that in case of an unexpected impact, the mirror will readily break away from a supporting base member.
2. Description of the Prior Art
When a rear view mirror is undetachably fixed within, for example, the dirver's chamber of an automobile and the automobile is stopped suddenly for any reason, the driver or a passenger may hit the rear view mirror causing the mirror structure to break and/or personal injury to occur.
Accordingly, it is desirable to provide a rear view mirror device which functions such that at the time a considerable force is applied thereto, the device will readily break away from its base without being broken. It is well known, for example, to attach the supporting arm of a rear view mirror to the body of a motor vehicle via an easily breakable base member.
One such known apparatus is disclosed in Japanese Utility Model Publication No. Sho 45-28017 published on Oct. 28, 1970. In this apparatus, the supporting arm is fixed by bolts and nuts to a portion of a base member made of a synthetic resin designed to be attached to the body of an automobile. The marginal portion of this base member around the portion thereof to which the supporting arm is fixed is so formed as to have a lessened thickness. Whenever a certain magnitude of force is applied to this mirror device, said marginal portion of the base member will fracture, and accordingly the mirror and its supporting arm, together with that portion of the base member to which the arm is fixed, will break away from the body of the automobile.
The aforesaid known arrangement has the advantage that by adjusting the thickness of the thin marginal portion surrounding the region to which the supporting arm is fixed, the resistance of this thinned marginal portion against destructive force may be varied. However, in order to determine a required thickness for each type of base member material in order to obtain a desired resistance against destructive force, it is necessary to prepare, in the stage of trial manufacture, various dies for molding thinned portions of widely varying thickness. This leads to a high cost of trial manufacture. Furthermore, when a rear view mirror device attached to a base member and having a thin marginal portion is in use, the presence of this thinned portion tends to cause the base member to develop resonance with the vibration of the automobile carrying such device with the resultant drawback that a stationary reflection is not achieved. In this case where a prism mirror having a viewing field adjusting means is attached to the aforesaid base member, the vibration of the mirror becomes even more aggravated because of the fact that a considerable amount of weight is supported at the free end of the supporting arm in such a device. In the case of a break away type rear view mirror device another desirable function of the mirror assembly is that whenever a substantial amount of force is applied to the mirror in the direction of the wind shield, the mirror arrangement will break away before it hits the wind shield. In view of the fact, however, that with a known mirror device of the type just described, its support arm in general will pivot about a portion of the aforesaid thinned region momentarily, and thus the shearing force will not be distributed uniformly to the entire thinned region. As a result, the entire thinned region will not be destroyed instantaneously. The site which serves as a fulcrum for the pivotal motion of the arm, in particular, tends to act as a hinge for this motion of the arm. Thus, it is possible that the mirror device as a whole will not completely break away until the arm has been substantially displaced. In other words, with such a prior art arrangement the mirror device may hit the wind shield.
Another known arrangement for detachably securing a rear view mirror device within an automobile is shown in Japanese Utility Model Publication No. Sho 47-21793 published July 18, 1972. In this arrangement, a substantially cylindrical aperture is formed in the central portion of a base member used to attach the supporting arm of the mirror to the body of an automobile. A cylindrical nut is provided within said aperture. A plurality of thin plate-like ribs extending radially towards the outer wall of the nut from the inner wall of the aperture are provided to support said nut within the aperture. These respective members are molded as integral parts of the base member. The base member is fixed to the body of the automobile by bolts and nuts, and a bolt is screwed into the nut through a bolt-receving hole formed in the bottom of the supporting arm of the mirror device. Thus, the mirror is attached to the body of the automobile.
The securing arrangement just described behaves such that whenever substantial impact forces are applied to the mirror, the ribs are destroyed so that the mirror device breaks away with the nut. A desired resistance to detachment is obtained by varying the ribs either in thickness or in number. As in the case of the previously described prior art, the determinination of the required thickness and number of ribs necessitates preparation of various kinds of dies in the stage of trial manufacture which, in turn, is very costly. Also, as in the case of the prior art discussed above, resonance develops during use and the resonance becomes more intensive when a heavy prism mirror is mounted on the supporting arm. Thus, the viewability of such a mirror device is poor. Also, as stated above, the attachment of the devices requires two cumbersome steps., i.e., fixing the base member to the body of an automobile and then fixing the supporting arm to the base. Thus, attaching a rear view mirror of this type is inefficient.
A further arrangement for detachably securing a rear view mirror to the body of an automobile is proposed in U.S. Pat. No. 3,599,926, dated Aug. 17, 1971. This arrangement is similar to that disclosed in the aforesaid Japanese Utility Model Publication Sho 45-28017 with the exception that the end of the supporting arm is large enough to cover the fragile portion of the base member. As discussed with respect to Japanese Utility Model Publication Sho 45-28017, the difficulty with this approach is that it is difficult to manufacture an attachment which will have sufficient thickness to prevent the rear view mirror from seriously vibrating while at the same time allowing it to break away when subjected to a reasonable force.